1. Field of the Invention
The present invention relates to an electric power-steering control apparatus that assists a steering force by means of a driving force from a motor.
2. Description of the Related Art
Generally, an electric power-steering control apparatus detects a steering torque generated when a driver rotates a steering wheel and calculates a current command for a motor that is connected to a steering column that links wheels with the steering wheel via a steering gear in a compensation calculation unit based on the detected steering torque. Then, a motor torque is generated to assist a steering operation by the driver by controlling a motor current to follow the current command.
In the compensation calculation unit, the current command is calculated by using gains, various filters, and the like for not impairing the stability and the road feel of a vehicle. To achieve such a purpose, also a calculation to change properties of the gain and the filter depending on a speed of the vehicle (vehicle speed) and a measured amplitude of the torque is generally performed. In most cases, it is necessary to optimize properties of the electric power-steering control apparatus according to the type of a vehicle to which the electric power-steering control apparatus is applied, and performing the optimization simply and systematically is an important issue. Furthermore, to realize the optimization using a low-cost calculation device, having the calculation as simple as possible is another important issue.
According to an electric power-steering control apparatus described in Japanese Patent Application Laid-open No. H7-309250, a signal is generated using a function called an assist curve, which is nonlinear for the magnitude of the steering torque and changes its characteristic depending on a vehicle speed. Then, the calculation of the current command is performed by applying a lag-lead type filter called an adaptive torque filter on a calculation result of the assist curve.
The assist curve is for realizing the road feel according to the magnitude of the steering torque and the vehicle speed, and because it has nonlinear characteristics as described above, a fluctuation occurs in a linearized gain that is a ratio of differential changes of an input and an output. Such an adaptive torque filter is designed in such a manner that a desired characteristic is obtained when the linearized gain of the assist curve is maximized; however, if the characteristic of the adaptive torque filter is fixed, the gain-crossover frequency (a frequency at which a gain of an open-loop frequency response crosses with 0 dB) that greatly affects the stability of a control system fluctuates, and the characteristic of the control system is deviated from a desired operation.
To cope with the problem, in a technology described in Japanese Patent Application Laid-open No. H7-309250, the linearized gain that is the ratio of the differential changes of the input and the output is successively calculated and a pole of the adaptive torque filter is successively calculated according to a result of calculating the linearized gain, so that the total gain of the compensation calculation unit does not change in the gain-crossover frequency. For this reason, due to the successive calculations of the linearized gain and the pole of the adaptive torque filter, there is a problem that the calculation amount is increased. Furthermore, because the assist curve described above is set for determining a steady-state characteristic of the compensation calculation unit, which the gain in the low frequency range, the characteristic of the adaptive torque filter is changed in the high frequency range, so that the crossover frequency that is the characteristic of the high frequency range is not changed with a change of the linearized gain of the assist curve. Therefore, a design for the low frequency range and a design for the high frequency range greatly interfere with each other. As a result, it is hard to figure out a correspondence between configurations of constituent units that configure the compensation calculation unit, such as the adaptive torque filter and the assist curve, and the designs for the frequency ranges described above, and the design is neither systematic nor simple.
In the electric power-steering control apparatus described in Japanese Patent Application Laid-open No. H7-309247, another countermeasure technique is described for a similar problem as the case in Japanese Patent Application Laid-open No. H7-309250. In this technology, for the detected steering torque, the signal is separated into a low frequency component and a high frequency component using a blending filter that is a combination of a low pass filter and a high pass filter. For the low frequency component of the torque detection value, a nonlinear calculation as in the assist curve described in Japanese Patent Application Laid-open No. H7-309250 is performed, and for the high frequency component of the torque detection value, a stationary linear gain is multiplied; and by inputting a sum of the calculation values to the adaptive torque filter, the output of the adaptive torque filter is calculated as the current command.
The adaptive torque filter described in Japanese Patent Application Laid-open No. H7-309247 is designed as a lag-lead filter such that the steering control apparatus shows a desired characteristic when the linearized gain is maximized as in the technology described in Japanese Patent Application Laid-open No. H7-309250. In other words, the gain in the low frequency range is designed to be relatively larger than the gain in the high frequency range. While the adaptive torque filter is changed according to the change of the linearized gain in the assist curve in Japanese Patent Application Laid-open No. H7-309250, in Japanese Patent Application Laid-open No. H7-309247, a desired characteristic, such that the gain-crossover frequency does not fluctuate with the change of the linearized gain, is realized by adding an appropriately designed blending filter with the characteristic of the adaptive torque filter fixed with respect to the change of the linearized gain. More specifically, the blending filter added to the adaptive torque filter is mainly designed to have a phase lead characteristic to decrease the gain in the low frequency range. As a result, the gain in the low frequency range, which greatly affects the road feel, is determined by the product of the adaptive torque filter and the blending filter, once amplified relatively largely by the adaptive torque filter and then decreased by the blending filter. For this reason, the design becomes complicated calculation amount is increased.
As described above, according to the electric power-steering control apparatus described in Japanese Patent Application Laid-open No. H7-309250, because the pole of the adaptive torque filter is successively calculated by successively calculating the linearized gain of the changing assist curve in the compensation calculation unit, there is a problem that the calculation amount is increased. Furthermore, it is hard to figure out the correspondence between the operation of each unit configuring the compensation calculation unit and the design that differs depending on the frequency range, and the design is neither systematic nor simple.
Furthermore, according to the electric power-steering control apparatus described in Japanese Patent Application Laid-open No. H7-309247, an adaptive torque filter, which is a lag-lead filter designed to be optimal in a specific condition of the linearized gain of the changing assist curve is provided, and an appropriately designed blending filter is further added such that the steering control apparatus has a desired characteristic according to the assist curve. For this reason, an order of the steering control apparatus is increased due to the addition of the blending filter, and the design becomes complicated accordingly.
In short, in the technologies described in the above literatures, there are problems that the calculation amount in the steering control apparatus is increased and that the design is neither systematic nor simple.